Prior to the introduction of the exchanged crystalline zeolites of the faujasite type, as a catalyst for cracking of hydrocarbons, a commonly used catalyst was composed of a silica-alumina cogel containing from about 3 to about 25 percent by weight of Al.sub.2 O.sub.3 on a volatile free basis.
Because of the substantially higher activity of the catalyst formed from crystalline zeolite, they have replaced to a large measure the silica-aluminum cogels as the primary component of a hydrocarbon conversion catalyst.
The literature relating to silica-alumina gels and their catalytic properties is extensive (see Iler, "The Colloid Chemistry of Silica and Silicates", Chapter VI, Cornell Press (1955) Ed.) and citations there given. See also, Erickson U.S. Pat. No. 2,872,410; Winyall U.S. Pat. No. 2,886,512; Wilson U.S. Pat. No. 3,124,541; Magee, et al., U.S. Pat. No. 3,433,748; Haden U.S. Pat. No. 3,065,054.
Maher, et al., U.S. Pat. No. 3,423,332 states that they produce an improved cracking catalyst from a "commercial silica-alumina" gel catalyst containing 13% Al.sub.2 O.sub.3. The "commercial catalyst" is "suitably" formed by gelling sodium silicate solution with CO.sub.2 and adding aluminum sulfate. The product is said to be activated by digesting the "commercial silica-alumina" catalyst with sodium hydroxide at a high pH at an elevated temperature to produce an amorphous product. The sodium content of the product is reduced by treatment with cations such as Ca, Mg, NH.sub.4 and rare earth cations.
Lussier, et al., U.S. Pat. No. 3,974,099 forms a silica-alumina cogel from a mixture of sodium silicate and sodium aluminate. The resultant gel is acidulated. Alumina is precipitated by reaction with the excess sodium aluminate in the reaction mixture. The dried gel is exchanged with ammonium sulfate to reduce the Na.sub.2 O content to less than 1%.